Natural gas conditioning process



Dec. 29 1942 B. MILLER f I NATURAL GAS' CONDITIONING PNocEss Filed Sept.23; 1940 BEM/AMW .M/LL 5R Patented Dec. 2,9, 1942 NATURAL GascoNm'rIoNnvG PnocEss a Benjamin Miner, Richmond nui, N. Y., signor toCities Service Oil Company, New York, N. Y., a corporation ofPennsylvania Application September 23, 1940, Serial No. 358,038

2 Claims. (Cl. 62-1755) v 'Ihis invention relates generally to theseparation of liquefiable hydrocarbons from natural gas, and isparticularly directed to an improved method for preparing natura.' gasproduced under `very high pressure from so-called distillate wells, forpipeline transportation. Most natural gas as produced carries a certainproportion of vapors of lqueiiable hydrocarbons which must be removedbefore transporting the gas through a pipeline in order to avoidinterference with the normal operation of the line by condensationtherein, and in order to avoid the danger of the presence of such liquidhydrocarbons -in Athe sas at the point of consumption.

` Natural gas is generally treated at or near the point of productionfor the recovery of liqueflable hydrocarbons including naturalgaslgasoline.

Natural gas gasoline may be recovered from natural gas in certainlocalities and at certain times because of its value as a liquid fuel.and such recovery may be profitable even though the natural g'as fromwhich the gasoline is stripped has no available market and must beeither discarded or returned to the underground reservoir from which itwas originally produced. On the other hand it may be necessary toextract liquenable hydrocarbons including Vnatural gas. gasoline fromgas which is to be transported through a pipeline even though the costof extracting such liqueiiable. hydrocarbons is greater than the valueof the material extracted. The expense of such extraction may only bewarranted by the necessity of producing a gas which can be transportedin pipelines and Autilized without inconvenience or hazard.

The conventional method for separating liqueilable hydrocarbons fromnatural gas which 4ig; to be transported by pipeline is the so-calledabsorption process which involves reducing the' pressure of the gas tothat-at which the gas is to be introduced into the pipeline, and thenprocess'- ing the gas by oil solvent absorption for the re covery ofliqueable constituents. 'I'he oil absorption process is very efcient forseparating from natural gas substantially'all of the butane and higherhydrocarbons,v and an appreciable traction of isobutane 'and propane.vhowever,` a plant for practicing the absorption process in- A lvolves alarge capital expenditure, and can only be justified economically whenthere is va very large volume of natural gas to be treated, or when thenatural gas to be treated carries a high y 'content of marketablenatural gas gasoline. In general the market value of natural gasgasoline Y at the producing `point is so vlow that it is un- 65economicalfrom the standpoint ofthe gasoline recovery alone to process agas containing only a few tenths of a gallon of natural gas gasoline per1000 cu. ft., even though several hundred million cubic feet of suchgasare availableper day for treatment. .On the other han'd, it isgenerally uneconomical to treat the gas from individual wells or` fromsmall groupsof wells in `a producing area, even though such gas maycontain asmuch as 1/2 to 1 gallon per 1000 cu. ft.,

A where only a few million cubic feet perday of such gas are availablefor treatment.

vWithin the last few years several so-called fdistillate" natural gasfields have been discovered in which the gas is produced under extremelyhigh pressure of the order, of 3000 pounds per square inch. Ii naturalgas followed the simple gas laws, such gas is produced at 3000 pounds-gas pressure would carry so little liquei'lable hydrocarbon that itwould not have tov be treated for pipeline transportation. However, ithas been knownlfor sometime that thehydrocarbon mixture which is presentin ordinary natural gas does not conform to the simple gas laws atpressures ranging yupwardly from a minimum of 500- 1000 lbs. per squareinch, depending upon the rtemperature and composition of the gas.- .The

pressure range within which natural gas does not follow the ordinary gaslaws is called the retrograde condensation range." Within the range ofretrograde condensation the high pressure natural gas may contain aconsiderable quantity 'of liquei'lable hydrocarbons, and thesehydrocarbons can be condensed by simply lowering the,

pressure of the gas within the range of .retrograde condensation,without lowering the gas temperature. On the other hand, at the lowpressure of only 300 to 500 pounds at which natural gas is usuallyintroduced to a transportation pipeline, thc. natural gas from so-calleddistillate fields follows the simple gas laws. In other words, naturalgas at these .lower pressures, on-` ,having its pressure further reducedwhile maintaining the temperature constant, will evaporate natural gasgasoline hydrocarbons in contact therewith.

Thus natural gas which is under pressure within the retrogradecondensation range behavesin a reverse manner'to the same gas at lowerpressure where the simple gas laws apply, and this phenomenon ofretrograde condensation as applied tohigh pressure natural gas, has beenused as the basis of another process for treating natvural gas from highpressure fields which is not to vbe subjected to pipelinetransportation, but which .According to this second process liqueiiablehydrocarbons are condensed from the high pressure gas by subjecting thegas to a partial lowering of temperature, with some reduction ofpressure but not below the retrograde condensation limits. Thethus-'treated gas is then returned to the underground reservoir fromwhich it came or to another underground reservoir. While the recovery ofliquid by this second recycle process is not as great as it would bewere the absorption method employed, it nevertheless may be moreprofitable because of the much lower capital investment for theequipmentnecessary for its practice. The process is based on thephenomenon of retrograde condensation, and is practiced within the rangeof pressures whereu an increase in separation of liquid occurs with apressure drop as well as witha temperature drop.

The second process of separating natural gas gasoline from gas which isto. be recycled (above referred to) is necessarily not as eiiicient asvthe absorption process now generally used for separating liqueablehydrocarbons from gas which is to' be transported by pipeline. Onereason for this is that the maximum separai tion of -liqueablehydrocarbons from naturalv aaoacsa that heretofore eected by applicationof the retrograde condensation phenomenon to the treatment of gas whichAisi to.be recycled.

With the foregoing and other objects and features in view. the inventionconsists of the improved process for conditioning natural gas forpipeline transportation which is hereinafter described and morevparticularly defined by the accompanyingclaims.

'Ihe process will now be described with reference to the accompanying owsheet which' gas under high pressures within the retrograde condensationrange. Such gas as produced from a well l0 carries a temperaturesubstantially exceeding atmospheric temperature. The ilrst step of theprocess is therefore to cool the gas to normal atmospheric temperature,or to a tem-v perature which can be attained by available air gas by aprocess based on the retrograde condensation phenomenon would occur atsuch a low pressure as to involve unduly expensive com. pressionequipment for recompressing the -gas to the recycling or reservoirpressure. 'I'hus if the reservoir pressure is assumed to be 3000 lbs.per square inch, and if the pressure at which maximum separation ofliqueiiable hydrocarbons be in the neighborhood of 900 lbs. per squareinch at the reservoir temperature, it would probably be uneconomical tooperate the `separating chamber of the recycle treating process -at aand water cooling, as in cooler I2. In order to avoid formation ofnatural gas hydrates during this preliminary cooling operation as aresult of the presence of moisture in the gas, it is usually desirableto add to the gas before cooling it a hydrate inhibitor such as methylalcohol, or a dehydrating agent such as a concentrated iaqueous solutionof calcium chloride.

After passing through the preliminary cooling step, the gas is subjectedto a refrigeration pressure substantially lower than say 150.0 lbs. v

per square inch; thisfor the reason that the retrograde condensate doesnot increase so, rapidly as does the power required for .recompress--ing the gas as the pressure decreases. In order to increase therecovery of retrograde condensate without undue pressure drop, recoursemay be-had to refrigeration. The' actual quantity of condensate may beincreased by refrigeration, but the relative quantity is very frequentlydecreased .because of the fact'thatin general the cooling operation bypassing it through a gas- -to-gas heat exchanger Il in indirect heattransfer relation to previously expanded gas on its way to the inlet Itof the transportation pipeline. The thus refrigerated gas is thenconductedlies in the range between 600 and 800 lbs. per n square inch,at temperatures in the neighborhood of 10 to 50 F. 'I'he expansion isprefera`b1y carried out in a chamber equipped with bailles andv ofsuillcient size to' allow for complete condensation of liqueflablehydrocarbons and pressure at which maximum condensation takes placedrops proportionately with any drop in temperature. Theprimary object ofthe present invention mechanical separation 'of the condensed liquidsfrom the expanded gas within the expansion chamber. Liquids thuscollected in the bottom of the expansion chamber can be separated bysettling into two layers, an aqueous layer oi' hyis to provide animproved economical process for separating liqueilabie hydrocarbons fromnatural gas produced in high pressure distillate wells as .a preliminaryto pipeline transportation. It will be understood that the process of`the present invention has particular appli-460 cation and utility forstripping natural gas from distillate wells of its liquid hydrocarboncontent so as to condition it for pipeline transportation, in caseswhere the amount and value of liquid condensate recoverable may beso lowand the amount of gas available for treatment at one point may be sosmall, comparatively, that application of the absorption process wouldbe less economical. By application. o f the present process, liqueflablehydrocarbons may be separated from the gas in amount sufficient to puiit in safe condition for transportation and use. Thus the presentprocess eects a much more complete separation of liquid from the gasthan drate inhibitor and a layer of unstabilized natural gas gasoline.

From the top of the expansion chamber the thus partially expanded gas isconducted past a second expansion valve 22 which is set to allowfurther'expansion of the gas-to thepressure at which it is desired tofeed the gas to the inlet of the transportation pipeline. 'I'his secondexpansion -eilects'a further drop in temperature of the gas body, andthe expanded and cooled gas is` then passed through the gas-to-gas heatexchanger I4 in indirect heat transfer relation with -a fresh portion ofhigh pressure gas; the

finally expanded gas serving as the refrigeration means forcooling suchhigh pressure gas.

Since the pressure of the gas is reduced in the separator or expansionchamber 2l to the lower limit of the retrograde condensation pressurerange for the gas at the temperature there- -in obtaining, maximumcondensation of liqueilable hydrocarbons from the gas takes place suresuitable for pipeline transportation below the retrograde condensationrange. Since the aqueous and gasoline condensates which are removed fromthe gas in the expansion chamber have a relatively low temperature atthe point at which condensation takes place, these liquids may, ifdesired, be circulated througha liquid to gas heat exchanger 24 in heattransfer relation to the unexpanded gas on its way to the expansionchamber to obtain some additional refrigeration effect, beforeconducting the\conden sates to distillation equipment for stabilizationof the gasoline and for regenerationof the hydrate inhibitor.

By the present process the gas is expanded vin two stages. The firstexpansionoccurs in the liquid separator wherein the pressure drop isfrom the producing Well pressure down to the pressure of maximumretrograde condensation for the particular gas being treated at thetemperature obtaining in the separator. In general, the lower thattemperature is, the lower is the pressure at which maximum condensationoccurs. The additional temperature drop of the gas which takes placeduring the second stage of expansion from the pressure in the separatordown to pipeline pressure, is used' to produce additional coolingeffect, by interchange, on the gas undergoing cooling on its way to theseparator, and this additional cooling results in the recovery withinthe separator of an additional quantity of liquid hydrocarbon. Theprincipal advantage of this additional separation of liquid is that itsremoval from the gas'is frequently a determining factor in finallyconditioning the gas for pipeline transportation.

'I'he refrigerating eiect of the second stage of expansion fromseparator pressure down to pipe line pressure 'depends 'on the separatortemperature and on the separator pressure,- if the pipeline pressure isfixed. In general, the higher the pressure in the separator and thelower the temperature, the greaterwill be the cooling effect of furtherexpansion. "While the cooling effect of the second expansion is greaterif the separator o pressure is kept high, it is alsov the case that ahigh separator pressure will result in liqueilable hydrocarbon beingleft in the gas which could have been liqueed if a lower separatorpressure had been maintained. 'Ihere is, therefore, for any particulargas at a particular initial pressure and initial temperature, and forany-definite pipeline pressure, one best pressure at which to operatethe separator. This best separator pressure or optimum pressure is thatpressure which results in the maximum separation of liquid fromA the gasat the lowest separator temperature. It is possible in starting upoperation of the plant to determine the optimum separator pressure byfollowing the separator temperature. In starting operation of theprocess, the initial expansion stage is allowed to take place throughonly a relatively small pressure drop, a large. pressure drop beingtaken in the second expansion stage. In other words, the process isfirst started .up with a relatively high separator pressure. At

the beginning of operations, the separator temperature will fall andthen will become constant as equilibrium is reached. After the separatortemperature has become constant, the separator 5 pressure is reducedagain'and the separator temperature is again watched until it hasdropped to a constant equilibrium point. This method of ladjustment iscontinued with each increment of separator pressure drop 'being followedby a period of establishing temperature equilibrium. Eventually a slightdrop in separator pressure will result in an increase in separatortemperature. When this occurs, the indication is that the optimumseparator pressure has been passed. The separator pressure is thenincreased slightly 'to bring the separator temperature to a minimumpoint which indicates optimum separator pressure.

Having thus described the invention', what is claimed as new is:

l. In a process for separating liqueiiable hydrocarbonconstituents fromnatural gas during expansion of the gas from a high pressure within theretrograde condensation range to' a pressure suitable for pipelinetransportation below the retrograde condensation range, the-stepscomprising directing a stream of said gas from the well toward aseparating zone, refrigerating said stream, partially expanding therefrigerated stream down to approximately the lower limit of theretrograde condensation range at the thus lowered temperature andseparating the stream in said zone into a gaseous component and liquidhydrocarbons condensed in said expansion, further expanding theseparated gaseous component below 500 pounds per square inch to thedesired transportation pressure after removing it from the separatingzone, and effecting part of said refrigeration by passing the furtherexpanded gaseous component in heat exchange relation with lthe gasstream proceeding toward said sepinitial cooling operation by passingthethus expanded gas in heat exchange relation with a fresh portion of gasto be treated, and at the beginning of operations determining theoptimum pressure to be maintained in the liquid separating zone bygradually dropping the pressure of the first expansion stage inincrements, allowing the temperature in the separating zone to reachequilibrium after 'each increment pressure drop, and locating thepressure of maximum retrograde condensation as that below which atemperature rise occurs in the separating zone after lan incrementpressure drop.

' BENJAMIN MILLER.

